Recently there has been considerable interest within the detergents industry in the production of detergent powders having a relatively high bulk density, for example 550 g/l and above.
Generally speaking, there are two main types of processes by which detergent powders can be prepared. The first type of process involves spray-drying an aqueous detergent slurry in a spray-drying tower. In the second type of process the various components are dry-mixed and optionally agglomerated with liquids, e.g. nonionics.
The most important factors which determine the bulk density of the final detergent powder are the chemical composition of the slurry in the case of a spray-drying process, and the bulk density of the starting materials in the case of a dry-mixing process. Both factors can only be varied within a limited range. For example, the bulk density of a dry-mixed powder can be increased by increasing its content of relatively dense sodium sulphate, but this does not contribute to the detergency of the powder so that its overall properties as a washing powder will generally be adversely affected.
Therefore, a substantial increase in bulk density can only be achieved by processing steps which lead to densification of the detergent powders. There are several processes known in the art leading to such densification. Particular attention has thereby been paid to densification of spray-dried powders by post-tower treatment.
In his article in Seifen-Ole-Fette-Wachse (114, 8, pages 315-316 (1988)), B. Ziolkowsky describes a process for the continuous manufacture of a detergent powder having an increased bulk density by treating a spray-dried detergent composition in two-step post-tower process, which can be carried out in a Patterson-Kelly Zig-Zag .RTM. agglomeration apparatus. In the first part of this machine, the spray-dried powder is fed into a rotating drum, in which a liquid-dispersing wheel equipped with cutting blades is rotating. In this first processing step a liquid is sprayed on to the powder and is thoroughly admixed therewith. By the action of the cutters, the powder is pulverized and the liquid causes agglomeration of the pulverized powder to form particles having an increased bulk density compared to that of the starting material.
The bulk density increase obtained is dependent on a number of factors, such as the residence time in the drum, its rotational speed and the number of cutting blades. After a short residence time, a light product is obtained, and after a long residence time a denser product. In the second part of the machine, which is essentially a rotating V-shaped tube, the final agglomeration and conditioning of the powder take place. After the densification process, the detergent is cooled and/or dried.
An example of a non-tower route for preparing a high bulk density detergent powders given in the Japanese patent application 60 072 999 (Kao). This application discloses a batch process whereby a detergent sulphonic acid, sodium carbonate, water and optionally other ingredients are brought into a high-shear mixer, followed by cooling to 40.degree. C. or below, pulverizing with zeolite powder and granulating.
Although it is possible by means of one or more of the above-mentioned processes to prepare detergent powders having an increased bulk density, each of those routes has its own disadvantages. It is an object of the present invention to provide an improved continuous process for obtaining high bulk density detergent compositions, or components thereof, having a bulk density of at least 550 g/l. The process should especially be suitable for the large scale manufacture of such compositions.
We have now found that granular detergent compositions or components having a high bulk density may be prepared by reacting a liquid acid precursor of an anionic surfactant with a solid water-soluble alkaline inorganic material in a high-speed mixer/densifier, treating the material in a moderate-speed granulator/densifier, and finally drying and/or cooling the material. The heat of the neutralization reaction between the acid surfactant precursor and the alkaline material is thereby used to bring the starting material into a deformable state, which was found to be necessary for obtaining a densification of the detergent composition.